It is often desirable to coat a substrate with a coating material, such as ink or an adhesive, in a desired location on the substrate. For example, pressure sensitive adhesive tape includes a web substrate coated with a layer of pressure sensitive adhesive. Similarly, POST-IT brand repositionable notes, available from the Minnesota Mining and Manufacturing Company of St. Paul, Minn., include a substrate having a band of pressure sensitive adhesive coated over a portion of the substrate. In these and other fields, it is desirable to apply the coating material to the substrate in a controlled manner.
One process of applying a coating material to a substrate is known as gravure coating, and is illustrated schematically in FIG. 1. The process includes a supply 12 of a substrate 10, an application roller 14, and a supply of coating material 18 through which the application roller is drawn. Also shown are a backing roller 19 that opposes the application roller, a doctor blade 21 for wiping excess coating material from the application roller, and a winding system having one or more rollers 13 to draw the substrate between the application roller and the backing roller. As shown in FIG. 2, the application roller includes a peripheral surface 30 having a multitude of individual cells 32 that are recessed from the peripheral surface of the application roller. The cells may be arranged in any pattern, as shown in FIG. 3, wherein cells 32 are formed in the peripheral surface at certain locations, and no cells are formed in locations 34.
The cells collect the coating material as the application roller passes through the supply of coating material, and thus the areas of the peripheral surface without cells (e.g. locations 34) do not collect coating material. If any residual coating material collects on the peripheral surface of the application roller, doctor blade 21 wipes that material from the peripheral surface prior to contact with the substrate.
When the substrate passes the application interface between the application roller and the backing roller, the material is drawn out of each of the cells because the coating material has a greater affinity for the substrate than for the application roller. The surface speed of the application roller is matched to the speed of the substrate, to enable complete removal of the coating material from the individual cells. If the surface speed of the substrate is greater than or less than that of the surface of the application roller, the cells of the application roller will not be entirely evacuated. Incomplete evacuation of the cells is undesirable, because the predetermined amount of coating material has not been transferred to the substrate.
Gravure coating, while having its own utility, is not easily modified to enable a thinner or thicker layer of coating material to be applied to the substrate. Because the cells on the peripheral surface have a fixed size and shape, and because the speed of the application roller and the substrate are matched, a particular application roller consistently applies the same pattern and thickness of coating material to the substrate. To change the pattern or thickness of coating material that is applied to the substrate, the application roller must be removed and replaced with a coating roller having different surface characteristics (e.g. more or less cells, greater or smaller spacing between adjacent cells, or deeper or more shallow cells). The coating process must be halted while a new application roller is attached to the coating apparatus, and roller replacement is therefore costly and undesirable. It would therefore be desirable to provide a method and apparatus for applying a coating material in different amounts and in varying patterns, without having to replace the application roller.
Planographic coating is similar to gravure coating in some regards, and includes such coating methods as flexography, lithography, and both wet and dry offset coating. A notable difference between gravure coating and planographic coating relates to the peripheral surface of the application roller, and the manner in which the coating material is carried on that surface. Whereas gravure coating uses an application roller having a plurality of cells that are recessed from the peripheral surface of the application roller, planographic coating uses a pattern roller 50 having a multitude of island portions 52 that are raised above peripheral surface 54, as shown in FIGS. 4 and 5. The coating material 56 is carried only on the outermost surface of each island portion 52, and the pattern roller 50 contacts the application roller 55 to transfer the coating material thereto. The application roller then transfers coating material 56 onto a substrate 58 in the desired pattern.
Although planographic coating also has certain benefits, it is difficult to alter the coating parameters quickly and inexpensively. To change the pattern or thickness of coating material applied to the substrate, the pattern roller must typically be replaced, because the characteristics of a particular pattern roller determine the pattern and thickness of coating material that will be applied to the substrate. Replacement of the pattern roller must take place when the coating process is stopped, and is therefore undesirable for the same reasons as stated above with reference to gravure coating. Furthermore, if several different pattern rollers must be available for each coating apparatus to provide a desired coating thickness or pattern, the investment in pattern rollers may be substantial.
It is therefore desirable to provide a method and apparatus for applying coating material to a substrate, wherein the coating characteristics may be altered without replacing the application roller.